Now you got the tax receipts as below from your employer, then you're not a contractor or 3.3% tax payer, and this is your turn to review it and check it whether it's fairly enough or missed some deductible items you expected to put in.
If some deductible items were missing, you may missed to submit it to your employer or you didn't joined at "HomeTax", so your employer can't access to your record.
You can still correct it manually, run to the bank you deal with and ask them to print the credit/check card statement(신용카드등 사용확인서), and try to get receipts for insurance premium, donation(including church, temple or any other religious org.).
If you had another income sources, like as an independent contractor, or any salary income from other employer, you need to file all of them combined and this is your responsibility and your liability.
Legally this filing is not your responsibility but employer's, so if it's wrongly filed, you can ask them to correct it, otherwise, you can still correct it by the end of May by yourself without any penalty.
Any questions, pls feel free to contact me, firstname.lastname@example.org